1. Field of the Invention
The present invention relates to a knob fitting structure, and more particularly to a knob fitting structure that can be suitably applied to a push-button switch.
2. Description of the Prior Art
To explain a drawing of a knob fitting structure according to the prior art, FIG. 5 shows an exploded perspective view of the knob fitting structure according to the prior art with the knob cut open.
A knob 51, consisting of a synthetic resin material and formed by molding, has a substantially rectangular top wall 51a, side walls 51b extending substantially vertically from the outer periphery and surrounding the four sides of the top wall 51a, and two pairs of fitting parts 51c extending inward (in the axial direction) from the central of the top wall 51a, and being opposite each other with some space between them and in parallel.
These pairs of the fitting parts 51c are provided with pairs of triangularly shaped engaging parts 51d positioned toward the respective tips away from the top wall 51a and protruding inward to be opposite each other, and the pairs of engaging parts 51d have slopes 51g inclining from the rear end toward the fore end of the fitting parts 51c. 
Between the pairs of the fitting parts 51c arranged opposite and in parallel to each other, there is formed a space 51e, and this space 51e communicates with the outside through a groove 51f provided between the pairs of engaging parts 51d. 
These pairs of fitting parts 51c are so configured as to be elastically deformed in the widthwise direction of the fitting parts 51c when they are in this state.
A fitting member 52, consisting of a synthetic resin material and formed by molding, has a rectangular base 52a and a quadrangular prismatic operation shaft 52b protruding (extending) outward (in the axial direction) from a front face 52c of the base 52a. 
The base 52a also has, in addition to the front face 52c, a rear face 52d opposite the front face 52c and side faces 52e surrounding on four sides the space between the front face 52c and the rear face 52d. Though not illustrated here, this base 52a is so configured as to be able to hold a movable contact.
The operation shaft 52b also has a pair of first side faces 52f on one pair of opposite sides, a pair of second side faces 52g on the other pair of opposite sides, a pair of trapezoid engaging parts 52h protruding outward from prescribed positions on the pair of first side faces 52f, and a tip 52m. 
The pair of engaging parts 52h provided on the first side faces 52f are smaller in width on the front side than on the rear side, has a slope 52j, and is formed in a trapezoid having a prescribed thickness.
This fitting member 52 is fitted in a state in which the operation shaft 52b within the space 51e provided between the pairs of fitting parts 51c of the knob 51.
In this state, the pair of engaging parts 51d provided on each pair of fitting parts 51c become engaged with the rear end of the pair of engaging parts 52h provided on each of the paired first side faces 52f of the operation shaft 52b to fit the knob 51 to the fitting member 52.
Next will be described how this knob 51 is fitted to the fitting member 52.
First, in a state in which the engaging parts 51d are opposite the engaging parts 52h, the operation shaft 52b is inserted into the space 51e surrounded by the two pairs of fitting parts 51c with the tip 52m of the operation shaft 52b ahead.
In this way, when the engaging parts 52h are positioned in the groove 51f, the slope 52j and the slopes 51g hit each other. When the operation shaft 52b is further inserted in this state, the pair of engaging parts 52h causes the pairs of engaging parts 51d of the pairs of fitting parts 51c to shift in the widthwise direction of the fitting parts 51c, and the pairs of fitting parts 51c are elastically deformed outward, i.e. in their widthwise direction. When the pairs of engaging parts 51d ride over the pair of engaging parts 52h, the fitting parts 51c are released from the suppression by the engaging parts 52h and return to their original positions, the engaging parts 51d are thereby engaged with the rear end of the engaging parts 52h to complete their incorporation.
At this time, the tip 52m of the fitting member 52 is in contact with inner surface of the top wall 51a of the knob 51 to arrange the knob 51 onto the fitting member 52.
In this knob fitting structure according to the prior art, however, the engaging parts 52h, when they are fitted to the fitting parts 51c, are forced aside by the engaging parts 52h in the widthwise direction, the fitting parts 51c are elastically deformed in the widthwise direction at the root to permit this forcing aside.
This results in the problem that the fitting parts 51c, as they are deformed in the widthwise direction in a strained way, suffer cracks or fractures at the root, and this strained deformation makes it impossible to ensure sufficient fitting strength for the fitting parts 51c. 
There is another problem that the fitting parts 51c have to be formed to have a long span to prevent cracks at the foot, inviting an increased fitting space in the axial direction.
An object of the present invention, intended to solve the problems noted above, is to provide a knob fitting structure which allows its knob to be stably fitted to the operation shaft and excels in assembling ease.